1. Field of the Invention
The present invention relates generally to a solid state laser, more specifically to a solid state laser pumped by a laser diode (abbreviated as LD below) as a pumping light source. Such solid state laser is excellent in both pumping efficiency and pumping distribution as well as easy in aligning the crystal axis even in an anisotropic laser medium.
2. Description of Related Art
LD-pumped solid state lasers using a LD as a pumping light source attract considerable attention as compact, highly efficient and longer lifetime laser light sources as compared to lamp-pumped solid state lasers. In particular, performance and quality of a LD have been improved significantly in recent years, thereby accelerating practical applications of such LD-pumped all solid state lasers. Such all solid state semiconductor lasers extend lifetime to 10,000 hours as compared to 1,000 hours of conventional lamp-pumped semiconductor lasers. Additionally, since the absorbing wavelength of the solid state laser medium and the oscillation wavelength of the LD are equal to each other in the LD-pumped solid state lasers, laser crystals can be pumped highly efficiently. High quantum efficiency helps to decrease thermal lens effect, thereby significantly improving the beam quality. As for pumping methods, various types of solid state lasers have been proposed and made by utilizing the so-called side surface pumping method in which the LD is disposed in parallel with the side surface of the light axis of the oscillating light from the elongate solid state laser medium.
Widely known as the laser media to be utilized for industry-use solid state lasers are Nd:YAG crystals and Nd:YVO4 crystals. Nd:YAG crystals feature high mechanical strength and excellent thermal conductivity, but are not suitable for oscillation at high pulse repetition rate over 30 kHz because of longer fluorescent lifetime. Additionally, Nb:YAG crystals are easy to handle because they have no crystal axis characteristic, thereby exhibiting no dependence to the polarizing characteristic of the pumping light in pumping efficiency. On the other hand, Nb:YVO4 crystals are inferior to Nb:YAG crystals in mechanical strength and thermal conductivity but have shorter fluorescence lifetime and higher induced emission cross sectional area than Nb:YAG crystals, thereby making them more suitable material for oscillating at higher pulse repetition rate. However, since Nb:YVO4 crystals have a single crystal axis, when used for laser oscillation, they not only oscillate in polarized in a particular orientation but also exhibit anisotropy in light absorbing characteristic. Since LDs tend to oscillate generally in polarized in a particular orientation as described hereinabove, the following Reference Patent 1 discloses that high pumping efficient can be achieved by choosing the orientation of polarization with respect to the crystallization.
Reference Patent 1
Japanese Un-Examined Patent Publication No. 4-137775
An example of the side surface pumping type of LD-pumped solid state laser is disclosed in the following Reference Patent 2.
Reference Patent 2
Japanese Un-Examined Patent Publication No. 2-54588
As shown in FIG. 6, the laser comprises a reflection layer 64 for enclosing the circumference of a cylindrical laser medium 61 along its optical axis and an opening 63 in a part of the reflection layer 64 for directing a pumping light 62 from a LD 60 into the laser medium 61. The reflection layer 64 covers almost the entire outer surface of the laser medium 61 except a small portion for directing the pumping light 62 from the LD 60. In this way, the pumping light 62 from the LD 60 is entrapped inside the laser medium 61 for efficient absorption of the pumping light 62 in the laser medium 61, thereby improving efficiency of oscillation energy.
As an example of the LD-pumped solid state laser for fixing the laser medium for proper alignment of the crystal axis of the laser medium in such a manner not to focus the reflection light onto a single point, there is proposed one using a rectangular rod shaped laser medium as shown in FIG. 7. The LD-pumped solid state laser comprises a reflection layer 74 covering the outer circumference of a rectangular rod shaped laser medium 71 along its optical axis and an opening 73 formed in a part of the reflection layer 74 for allowing a pumping light 72 from an LD 70 to enter the laser medium 71. Most of the outer surface of the laser medium 71 is covered by the reflection layer 74 except a small portion for receiving the pumping light 72 from the LD 70. In this way, the pumping light 72 from the LD 70 is entrapped inside the laser medium 71 for efficient absorption of the pumping light by the laser medium 71, thereby improving efficiency of oscillation energy. The use of rectangular Nd:YVO4 crystal as the laser medium 74 enables to align the orientation of the optical electric field of the pumping light 72 in parallel with the c-axis of the Nd:YVO4 crystal that is cut along the a-axis.
An example of a solid state laser capable of uniformly pumping the laser medium without using a focusing optical system is disclosed in, for example, the following Reference patent 3.
Reference Patent 3
Japanese Un-Examined Patent Publication No. 7-307510
As shown in FIG. 8, the solid state laser utilizes the particular feature of a laser medium 81 that exhibits a predetermined refractive index with respect to the pumping light. The laser medium 81 is disposed adjacent to a source of pumping light 82 and the laser medium 81 has a convex surface portion 86 at the side toward the pumping light source, thereby reducing the diffusing angle of the pumping light 82 without using a focusing optical system.
Also, an invention for effectively returning the pumping light not absorbed by a solid state laser rod (laser medium) back into the solid state laser rod is disclosed in the following Reference Patent 4.
Reference Patent 4
Japanese Un-Examined Patent Publication No. 2001-244526
Unfortunately, however, the solid state laser as disclosed in the Reference Patent 4 and shown in FIG. 9 is a multiple coaxial construction comprising a solid state laser rod 1, a gap for cooling medium 2, a cooling tube 3 and a cylindrical member 4 for providing a reflection surface. Accordingly, any pumping light not absorbed by the solid state laser rod 1 passes through the gap for the cooling medium 2 and the cooling tube 3 for at least twice, thereby attenuating the pumping light and failing to effectively utilize the reflection light.
However, all of the above mentioned conventional LD-pumped solid state lasers have a common problem that the laser medium can not be uniformly pumped by the LD.
For example, in the example as shown in FIG. 6, since the reflection layer 64 is circular, the reflection light tends to be focused on a single point, thereby not uniformly pumping the laser medium 61 by the LD (See FIG. 10). On the other hand, in the example as shown in FIG. 7, although the rectangular reflection surface helps to distribute the reflected light over the entire area rather than focusing on a single point, the dispersing angle of the pumping light 72 from the LD 70 tends to cause diffusion in pumping distribution that is weaker at the outer side of the laser medium 71, thereby causing high dimension spatial mode. As a result, the beam quality may be deteriorated as mentioned in the following Reference Patent 5. It is preferable that the laser medium is pumped by the pumping light in a parallel manner.
Reference Patent 5
Japanese Un-Examined Patent Publication No. 5-335662
Other problems associated with the conventional solid state lasers include difficulty in aligning the crystal axis of the laser medium and incapability of uniformly and efficiently pumping the laser medium.
For example, in the solid state laser as shown in FIG. 6, it is difficult to align the crystal axis of the laser medium 61 because it is cylindrical. In the solid state laser as shown in FIG. 7, the laser medium 71 is rectangular which is easy to align the crystal axis, but the pumping light 72 from the LD 70 tends to largely diffuse in the laser medium 71, thereby not uniformly pumping it. In the solid state laser as shown in FIG. 8, the laser medium 81 is convex in one surface but flat in the opposite surface on which a mirror is disposed. As a result, a part of the pumping light 82 passes through the output mirror 84, thereby not efficiently pumping the laser medium 81. Additionally, no attention is paid to the relationship between the crystal axis of the laser medium 81 and the pumping light 82.